This invention relates to an improved method of and apparatus for clarifying liquids and, in particular, to a filtration system utilizing horizontally disposed filters.
More specifically, this invention relates to a filtration system wherein horizontally disposed filter leaves, utilized to clarify a liquid, are automatically cleansed as required and supplied with new filter media for the continued operation thereof. Periodically the filter chamber is automatically drained and the expended filter media removed from the filter leaves for disposal or to recover the materials contained therein.
Clarification of liquids and/or the recovery of particulate materials from a liquid are required in many instances wherein it is desired to remove solid and/or chemical contaminants from the liquid prior to discharging the liquid, for example, into sewers. In other applications it is desired to recover materials suspended or dissolved in the liquid, such as the recovery of metals from solutions or the recovery of organic compounds in order to obtain desired constituents therefrom.
In certain other types of applications, it is necessary to recycle the liquid clarified by the filtration in order that it be continuously reused without discharge into sewage lines and/or subjected to costly methods of disposal. Due to requirements and regulations for environmental pollution control, or in other instances where the cost of the liquid is such that the disposal of the liquid would be uneconomical, the liquid must be recycled so that the filtrate can be continuously reused.
Various filtration systems have been developed for clarifying liquids by removing solid particles (whether initially dissolved or suspended therein) and chemical contaminants, as well as to recover reclaimable materials from the liquid. Certain of these systems employ mechanical screening devices utilized as a support for a chemically absorptive/adsorptive material such that the solid and the chemical contaminants are absorbed and/or adsorbed on the surface of the filter. However, it has been found that as the contaminants are removed from the liquid they accumulate on the surface of the filter inhibiting the flow of the liquid therethrough, eventually clogging the filter and necessitating the removal of the filter cake from the filter septum. Another type of filtration system utilizes a disposable filter that upon becoming clogged is removed from the system and another replaceable filter inserted.
In those filtration systems wherein a filter cake is formed on the filter septum, various filter cleaning means, such as brushes, scrapers and/or sprayers are used to mechanically remove the filter cake from the filter septum, or screen. Another method employed to remove filter cake from the septum is by a backwashing action utilizing a reverse flow of filtrate through the filter. However, the backwashing of filtrate through the filter is detremental to the filter and reduces its life considerably. Since most filters are constructed of a fine mesh screen septum supported solely on the low-pressure side by a support structure, this fine mesh screen distends and distorts easily when subjected to pressure from its low-pressure side.
Therefore, as previously indicated, the formation of the filter cake and its removal from the filter septum upon the accumulation of a predetermined amount of material from the liquid are important factors in the operation of all filtration systems.
Another important operational consideration for filtration systems is the orientation of the filter elements.
In certain applications vertically disposed filter leaves are utilized. While systems utilizing vertically disposed filters provide for ease of removal of filter cake from the septum upon the accumulation of material due to the assistance of gravity, it has been found that the vertical orientation of the leaves creates other problems. In applications wherein the flow rate of the liquid through the filter leaves is slow, the filter cake material has a tendency to prematurely fall from the septum. Therefore, in applications wherein the liquid flow rate is insufficient to create a pressure sufficient to hold the filter cake to the filter septum, a horizontally disposed filter must be utilized. In addition, the horizontal disposition of the filter not only prevents premature separation of the filter cake from the septum, but generally yields an improved filter cake in that during the coating or filter formation phase of operation a more uniform distribution of filter cake is formed on the septum. Furthermore, since the filter cake is formed in a horizontal position, imperfections in the cake such as cracks or holes are minimized.
In applications such as described above, a horizontally disposed filter leaf has many advantages over a filtration system wherein the filters are disposed vertically. However, since the horizontally disposed filter leaves are blind on the lowermost side, it is difficult to dispose of expended filter cake and replenish the filtration medium unless the entire contents of the filter chamber are drained causing a loss of the liquid and materials contained therein.
One type of prior art device employing a horizontally disposed filter rotates the filter leaves at a high rate of speed throwing the filter cake off the septum by centrifugal force. There are several disadvantages in this type of filtration apparatus in that the filter cake sometimes is strongly bonded to the filter septum such that upon the application of the centrifugal force the septum is ripped from the support materials along with the expended filter cake. In this event the filter must be shut down and the leaf replaced. In other instances, the filter cake is sufficiently bonded to the septum and the filter septum sufficiently secured to the support material such that the entire cake is not removed resulting in pieces of expended filter cake remaining attached to the septum which results in the formation of an uneven cake upon subsequent precoating.
Another disadvantage of prior art filtration systems utilizing horizontally disposed filters is that as the contents of the filter chamber are drained, an operator must control the draining of the filter chamber by sequentially closing valves connected to each of the filters. Since the lowermost portion of the filter leaf is blind, liquid will flow through the filter leaf and, as the liquid level is lowered, air will flow through the outlet of the uppermost leaf preventing the remaining liquid from draining from the tank. Therefore, an operator is required to sequentially close valves connected to each individual filter leaf to prevent air bleeding in order to enable the entire contents of the chamber to be withdrawn.
Since the utilization of the prior art filtering devices described above involves either difficulties in operation, such as maintaining a suitable precoat for filtration, or incurs substantial labor expense, none of these devices are completely satisfactory for general continuous operation of the filter apparatus and especially in those applications wherein it is extremely critical not to contaminant the filtrate once it has passed through the filtration system.